Actuator

Abstract

This makes it easy to reduce the axial error of the ball screw shaft. [Solution] The actuator comprises a hollow motor having a hollow rotating shaft, a bearing supporting the hollow rotating shaft, a ball screw nut that rotates together with the hollow rotating shaft around a central axis, a ball screw shaft that is driven in a straight line by the rotation of the ball screw nut, a ring that is kept stationary relative to the inner ring of the bearing, and a plurality of rotation restricting members that fix the flange of the ball screw nut to the ring and restrict the rotation of the ball screw nut relative to the ring. The relative angular position of the ball screw nut with respect to the ring is adjustable. The flange of the ball screw nut has a plurality of through holes or through grooves at angular intervals around the central axis, and the angular interval is less than 45 degrees. The ring has a plurality of insertion holes. Each of the rotation restricting members passes through either the through hole or through groove of the flange and is screwed into either the insertion hole of the ring.

Description

[Technical Field]

[0001] This invention relates to an actuator. [Background technology]

[0002] Patent Document 1 discloses an electric actuator that rotates the nut of a ball screw with a motor to move the ball screw shaft along its axis.

[0003] Patent Document 2 discloses a robot including a parallel link with multiple drive devices. Each drive device includes an actuator for moving a linear motion member. The actuator includes a ball screw mechanism for moving the linear motion member. The robot disclosed in Patent Document 2 has multiple degrees of freedom due to the parallel link mechanism being connected in series, and the configuration including the parallel link provides superior accuracy and strength compared to a typical serial link robot, and allows for miniaturization. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6632909 [Patent Document 2] Patent No. 7088440 [Overview of the project] [Problems that the invention aims to solve]

[0005] In robots with parallel links connected in series, the end-effector working point is located far from each actuator. Therefore, misalignment of the ball screw mechanism in each actuator significantly affects the positioning accuracy of the robot's end-effector movement. Consequently, the machining accuracy of the ball screw shaft in the actuator and the mounting accuracy between the ball screw mechanism and the linear motion member are crucial.

[0006] However, there are limits to how much the positioning accuracy of the linear motion member can be improved by increasing the machining accuracy of the ball screw shaft and the mounting accuracy between the ball screw mechanism and the linear motion member.

[0007] For example, consider a ball screw mechanism in which the ball screw shaft is moved linearly by the rotation of a ball screw nut. In a bearing that supports the rotating part connected to the ball screw nut, the rotating part flexes slightly due to errors in the roundness of the inner ring and the outer ring, to which the rotating part is fixed. Therefore, during one rotation of the ball screw nut and the rotating part, the inner ring moves a small amount axially relative to the outer ring, and the ball screw nut connected to the rotating part to which the inner ring is fixed also moves a small amount axially relative to the outer ring. The amount of movement of the inner ring and ball screw nut relative to the outer ring can be called the axial error of the ball screw nut caused by the bearing. Because the inner ring moves periodically relative to the outer ring, the axial error of the ball screw nut caused by the bearing fluctuates periodically (the period corresponds to one rotation of the ball screw nut). If there is an axial error of the ball screw nut caused by the bearing, the actual amount of movement of the ball screw shaft inside the ball screw nut will deviate from the ideal amount of movement.

[0008] Furthermore, in a ball screw mechanism having a ball screw shaft that is moved linearly by the rotation of a ball screw nut, the ball screw shaft deflects slightly due to misalignment and machining errors between the ball screw nut and the ball screw shaft. Due to this deflection, the ball screw shaft moves a small amount along the axial direction relative to the ball screw nut. Therefore, the actual amount of movement of the ball screw shaft during one rotation of the ball screw nut deviates from the ideal amount of movement of the ball screw shaft. This deviation can be called the axial error of the ball screw shaft caused by the ball screw mechanism. While the ball screw shaft is continuously moved, the axial error of the ball screw shaft caused by the ball screw mechanism fluctuates periodically (the period corresponds to one rotation of the ball screw nut).

[0009] Axial errors in ball screw nuts caused by bearings are due to errors in the roundness of the inner ring and the outer ring, making it extremely difficult to completely eliminate them. Similarly, axial errors caused by ball screw mechanisms are due to misalignment and machining errors between the ball screw nut and the ball screw shaft, making it extremely difficult to completely eliminate them.

[0010] The displacement of a ball screw shaft is affected by both the axial error caused by the bearing and the axial error caused by the ball screw mechanism. In other words, the displacement of the ball screw shaft is the sum of the axial errors caused by the bearing and the axial errors caused by the ball screw mechanism.

[0011] When the periodic axial error of the ball screw shaft caused by the ball screw mechanism is amplified by the periodic axial error of the ball screw nut caused by the bearing, the actual amount of movement of the ball screw shaft deviates significantly from the ideal amount of movement, resulting in a decrease in the positioning accuracy of the ball screw shaft and, consequently, the linear motion member.

[0012] Therefore, the present invention aims to provide an actuator that makes it easy to reduce the axial error of a ball screw shaft. [Means for solving the problem]

[0013] One embodiment of the actuator according to the present invention comprises a hollow motor having a hollow rotating shaft, a hollow rotor fixed to the hollow rotating shaft, and a stator arranged around the hollow rotor to rotate the hollow rotor and the hollow rotating shaft around a central axis; at least one bearing having an outer ring fixed to a housing and an inner ring fixed to the hollow rotating shaft, which rotatably supports the hollow rotating shaft; a ball screw nut having a cylindrical portion inserted into the hollow rotating shaft and a flange connected to the cylindrical portion, which is connected to the hollow rotating shaft and rotates together with the hollow rotating shaft around a central axis; a ball screw shaft engaged with the ball screw nut and driven in line by the rotation of the ball screw nut; a ring whose relative rotation with respect to the hollow rotating shaft and the inner ring of the bearing is restricted; and a plurality of rotation restricting members that fix the flange of the ball screw nut to the ring and restrict the rotation of the ball screw nut relative to the ring. The relative angular position of the ball screw nut with respect to the ring is adjustable around the central axis. The flange of the ball screw nut has a plurality of through holes or through grooves formed at angular intervals around the central axis, the angular interval being less than 45 degrees, and the number of through holes or through grooves is greater than the number of rotation restricting members. The ring has a plurality of insertion holes formed at angular intervals around the central axis. Each of the rotation restricting members passes through either of the through holes or through grooves of the flange and is inserted into either of the insertion holes of the ring to fix the flange of the ball screw nut to the ring.

[0014] According to an aspect of the present invention, the ring's relative rotation is restricted with respect to the inner ring of the bearing, and it moves periodically along the central axis, influenced by the periodic movement of the inner ring relative to the outer ring of the bearing. That is, the ring has a periodic axial error caused by the bearing. On the other hand, the ball screw nut is the reference position for the periodic axial error of the ball screw shaft caused by the ball screw mechanism. The relative angular position of the ball screw nut with respect to the ring around the central axis is adjustable, and the phase of the periodic axial error caused by the ball screw mechanism can be adjusted relative to the phase of the periodic axial error caused by the bearing. The flange of the ball screw nut is fixed to the ring by a plurality of rotation restricting members, restricting the rotation of the ball screw nut with respect to the ring. The flange of the ball screw nut has a plurality of through holes or through grooves, which are greater than the number of rotation restricting members, and the angular spacing between the plurality of through holes or through grooves is less than 45 degrees. Therefore, the phase of the axial error caused by the ball screw mechanism can be adjusted at angular intervals of less than 45 degrees (i.e., in multiple stages) relative to the phase of the axial error caused by the bearing. If the ball screw nut is fixed to the ring at an angular position where the periodic fluctuation of the displacement of the ball screw shaft relative to the ball screw nut (axial error caused by the ball screw mechanism) most effectively cancels out the periodic movement of the inner ring relative to the outer ring of the bearing (axial error caused by the bearing), then the total axial error applied to the ball screw shaft can be reduced.

[0015] The number of insertion holes in the ring may be greater than the number of rotation restricting members. The insertion holes may form multiple sets, and the insertion holes in each set may be arranged at equal angular intervals around the central axis, and the multiple sets may be arranged around the central axis at angular intervals different from the angular intervals of the through holes or through grooves. In this case, there is an option to insert the rotation restricting member into one of the multiple sets of insertion holes. Since the angular spacing of the multiple sets of insertion holes is different from the angular spacing of the through holes or through grooves, the ball screw nut can be adjusted to smaller angular intervals relative to the ring. Therefore, the total axial error can be further reduced.

[0016] A male thread may be formed at the end of the hollow rotating shaft, centered on the central axis, and the ring may have a female thread that is screwed into the male thread. When the male thread and the female thread are screwed together and pressed against the inner ring, the relative rotation of the ring with respect to the hollow rotating shaft and the inner ring may be restricted. In this case, the ring's internal thread is screwed onto the external thread at the end of the hollow rotating shaft, and it is pressed against the inner ring of the bearing, thereby restricting relative rotation between the hollow rotating shaft and the inner ring. Therefore, the ring can be kept in a non-rotating state without the use of other fasteners.

[0017] The actual amount of movement of the ball screw shaft along the central axis relative to the ball screw nut when the ball screw nut rotates differs from the ideal amount of movement of the ball screw shaft relative to the ball screw nut, and this difference between the ideal amount of movement and the actual amount of movement may fluctuate periodically with the rotation of the ball screw nut. As the hollow rotating shaft fixed to the inner ring of the bearing rotates, the inner ring of the bearing may move periodically along the central axis relative to the outer ring. The ball screw nut may be fixed to the ring at an angular position where the periodic fluctuation of the deviation in the amount of movement of the ball screw shaft most largely cancels out the periodic movement of the inner ring relative to the outer ring of the bearing. In this case, the ball screw nut is fixed to the ring at an angular position where the periodic fluctuation of the displacement of the ball screw shaft relative to the ball screw nut (axial error caused by the ball screw mechanism) most effectively cancels out the periodic movement of the inner ring relative to the outer ring of the bearing (axial error caused by the bearing). Therefore, the total axial error applied to the ball screw shaft can be minimized. [Effects of the Invention]

[0018] According to an aspect of the present invention, it is easy to cancel out as much as possible the axial error caused by the bearing and the axial error caused by the ball screw mechanism, and as a result it is easy to reduce the axial error of the ball screw shaft.

Brief Description of the Drawings

[0019] [Figure 1] Figure 1 is a front view of an actuator according to an embodiment of the present invention. [Figure 2] Figure 2 is a cross-sectional view of Figure 1. [Figure 3] Figure 3 is a perspective view of Figure 1. [Figure 4] Figure 4 is an exploded perspective view of the drive assembly of the actuator. [Figure 5] Figure 5 is a perspective view of a state where a part of the drive assembly is assembled. [Figure 6] Figure 6 is a front view of the ball screw nut of the drive assembly. [Figure 7] Figure 7 is a perspective view of the ring (lock screw) of the drive assembly. [Figure 8] Figure 8 is a front view of a combination of the ball screw nut of Figure 6 and the ring of Figure 7 fixed by a rotation restricting screw. [Figure 9] Figure 9 is a perspective view of a ring (lock screw) according to another embodiment. [Figure 10] Figure 10 is a front view of a combination of the ball screw nut of Figure 6 and the ring of Figure 9 fixed by a rotation restricting screw. [Figure 11] Figure 11 is a front view of a combination of the ball screw nut of Figure 6 and the ring of Figure 9 fixed by a rotation restricting screw. [Figure 12] Figure 12 is a graph showing the variation of the axial error of the ball screw nut caused by one bearing with respect to the rotation angle of the ball screw shaft. [Figure 13] Figure 13 is a graph showing the variation of the axial error of the ball screw shaft caused by the ball screw mechanism with respect to the rotation angle of the ball screw shaft. [Figure 14] Figure 14 is a graph showing the variation of the total axial error in which the axial error of Figure 12 is amplified worst to the axial error of Figure 13. [Figure 15]Figure 15 is a graph showing the variation in total axial error where the axial error in Figure 12 is most significantly offset by the axial error in Figure 13. [Figure 16] Figure 16 is a front view of the combination of the ball screw nut of the comparative example, fixed with a rotation-restricting screw, and the ring shown in Figure 7. [Figure 17] Figure 17 is a graph showing the variation in the amplified total axial error and the offset total axial error with respect to the displacement of the ball screw shaft. [Figure 18] Figure 18 is a front view of a ball screw nut according to another embodiment. [Figure 19] Figure 19 is a cross-sectional view of an actuator according to another embodiment. [Figure 20] Figure 20 is a cross-sectional view of an actuator according to another embodiment. [Modes for carrying out the invention]

[0020] Embodiments of the present invention will be described below with reference to the attached drawings. The scale of the drawings is not necessarily accurate, and some features may be exaggerated or omitted.

[0021] As shown in Figures 1 to 3, the actuator 1 according to an embodiment of the present invention comprises a drive assembly 10 and a slider assembly 40.

[0022] The drive assembly 10 comprises a housing 11, a hollow motor 12, a ball screw nut 18, and a ball screw shaft 20.

[0023] The housing 11 is a hollow body with a rectangular parallelepiped shape, enclosing the hollow motor 12 and the ball screw nut 18, and also enclosing a portion of the ball screw shaft 20 engaged with the ball screw nut 18. The housing 11 has a front housing piece 11a and a rear housing piece 11b, which are fixed to each other. For convenience of explanation, the left side of Figures 1 and 2 will be referred to as the front, and the right side as the rear. A cover 11c is fixed to the front end of the front housing piece 11a.

[0024] As shown in Figure 2, the hollow motor 12 has a stator 12a, a hollow rotor 12b, and a hollow rotating shaft 13. The hollow motor 12 is, for example, a stepping motor.

[0025] The stator 12a is nearly annular in shape and is positioned in the internal space of the rear housing piece 11b, and is fixed to the rear housing piece 11b.

[0026] The hollow rotor 12b is positioned radially inward of the stator 12a and is surrounded by the stator 12a. The hollow rotating shaft 13 is a cylindrical tube with a shape in which multiple annular sections with different outer and inner diameters are concentrically connected. The hollow rotor 12b is positioned around the rear portion of the hollow rotating shaft 13. The hollow rotor 12b is fixed to the hollow rotating shaft 13, and when the hollow rotor 12b is rotated around its central axis Ax, the hollow rotating shaft 13 is also rotated around the central axis Ax of the hollow rotor 12b. The stator 12a, positioned around the hollow rotor 12b, rotates the hollow rotor 12b and the hollow rotating shaft 13 around the central axis Ax.

[0027] A handle 14 is fixed to the rear end of the hollow rotating shaft 13. The handle 14 is located outside the housing 11. A worker manufacturing the actuator 1 can grasp the handle 14 and rotate the hollow rotating shaft 13 around the central axis Ax. The worker can also grasp the handle 14 and carry the drive assembly 10.

[0028] Bearings 15, 16, and 17 that rotatably support the hollow rotating shaft 13 are arranged in the internal space of the housing 11. The outer rings of bearings 15, 16, and 17 are fixed to the housing 11, and the inner rings of bearings 15, 16, and 17 are fixed to the hollow rotating shaft 13. Bearing 15 is located inside the rear housing piece 11b and supports the rear end of the hollow rotating shaft 13. Bearing 16 is located inside near the boundary between the front housing piece 11a and the rear housing piece 11b, and bearing 17 is located inside the front housing piece 11a. Bearings 16 and 17 support the front end of the hollow rotating shaft 13.

[0029] The ball screw nut 18 is positioned concentrically with the hollow rotating shaft 13 inside the housing 11. The ball screw nut 18 is concentrically connected to the hollow rotating shaft 13 of the hollow motor 12 and rotates together with the hollow rotor 12b and the hollow rotating shaft 13 around the central axis Ax by the hollow motor 12. The ball screw nut 18 has a cylindrical portion 18a that is inserted into the front end of the hollow rotating shaft 13 and a flange 18b integrally connected to the front end of the cylindrical portion 18a.

[0030] The ball screw nut 18 is concentrically connected to the hollow rotating shaft 13 and is restricted from moving in a linear direction along the central axis Ax. Details of the connection between the ball screw nut 18 and the hollow rotating shaft 13 will be described later.

[0031] The ball screw shaft 20 is engaged with the ball screw nut 18 and is moved linearly along the central axis Ax by the rotation of the ball screw nut 18. Thus, the ball screw shaft 20 is advanced in the forward direction, protruding from the housing 11, and retracted in the backward direction, moving toward the interior of the housing 11.

[0032] In this embodiment, the actuator 1 is miniaturized by concentrically arranging the ball screw mechanism, which has a ball screw shaft 20 and a ball screw nut 18, with the hollow motor 12.

[0033] A ring (lock screw) 22 is positioned around the front end of the hollow rotating shaft 13. The ring 22 is fixed to the front end of the hollow rotating shaft 13. Specifically, a male thread 13a is formed at the front end of the hollow rotating shaft 13, centered on the central axis Ax, and a female thread is formed on the inner circumferential surface of the ring 22 that screws into the male thread 13a. Furthermore, the rear end of the ring 22 (the small-diameter portion 22b, described later) is pressed against the inner ring of the bearing 17. Therefore, the ring 22 is restricted from rotating relative to the hollow rotating shaft 13 and the inner ring of the bearing 17.

[0034] Furthermore, the flange 18b of the ball screw nut 18 is fixed to the ring 22 by a plurality of rotation restricting screws (rotation restricting members) 24. As will be described later, the rotation restricting screws 24 engage with the flange 18b of the ball screw nut 18 and also engage with the ring 22, thereby restricting the relative rotation of the ball screw nut 18 with respect to the ring 22.

[0035] The cylindrical portion 18a of the ball screw nut 18 is inserted into the front end of the hollow rotating shaft 13, and the relative angular position of the ball screw nut 18 with respect to the hollow rotating shaft 13 and the ring 22 is adjustable around the central axis Ax. After adjusting the angular position of the ball screw nut 18, the flange 18b is fixed to the ring 22 with a rotation restricting screw 24. Thus, the rotation restricting screw 24 restricts the relative angular position of the ball screw nut 18 with respect to the hollow rotating shaft 13 and the ring 22 (and also with respect to the inner ring of the bearing 17). In other words, the rotation restricting screw 24 prevents the ball screw nut 18 from rotating around the central axis Ax and connects the ball screw nut 18 to the hollow rotating shaft 13.

[0036] The ball screw shaft 20 has a screw section 20a, an end section 20b, and a limiter mounting section 20c, all of which are concentric with each other.

[0037] The threaded portion 20a is engaged with the ball screw nut 18. The threaded portion 20a is inserted into the internal space of the hollow rotating shaft 13 of the hollow motor 12.

[0038] An external threaded portion 20d is formed at the tip of the end portion 20b of the ball screw shaft 20. A retaining ring 26, made of a rigid material (e.g., metal), is fixed to the end portion 20b. The method of fixing is not limited, but for example, the retaining ring 26 may be fixed to the end portion 20b by press-fitting the end portion 20b into the through hole in the center of the retaining ring 26.

[0039] The limiter mounting portion 20c is located between the screw shaft 20a and the end portion 20b. A limiter ring 28 is fixed to the limiter mounting portion 20c by a plurality of screws 27. The limiter ring 28 is made of a rigid material (e.g., metal). When the ball screw shaft 20 is retracted toward the interior of the housing 11, the limiter ring 28 comes into contact with the cover 11c, preventing the ball screw shaft 20 from retracting. In this way, the limiter ring 28 restricts the linear motion of the ball screw shaft 20 in the direction of the central axis Ax.

[0040] The mounting portion 44 of the slider 42, which will be described later, has a through hole 44b and a recess 44c. The male threaded portion 20d at the end 20b of the ball screw shaft 20 is inserted into the through hole 44b. A fastening nut 29 is placed inside the recess 44c, and the fastening nut 29 is screwed onto the male threaded portion 20d. Therefore, the mounting portion 44 is fastened to the ball screw shaft 20 by being sandwiched between the retaining ring 26 and the fastening nut 29.

[0041] The slider assembly 40 includes a housing 41, a slider (linear motion member) 42, a cover 45, and a plurality of linear guides 46.

[0042] The housing 41 has an elongated rectangular shape that extends parallel to the central axis Ax of the ball screw shaft 20 and is fixed to the housing 11 of the drive assembly 10 by a screw 47.

[0043] The slider 42 is formed from a rigid material (e.g., metal) and has an elongated rectangular bar portion 43 and a mounting portion 44 formed to protrude from one end of the bar portion 43 to the other, and as shown in Figures 1 and 2, it has an overall L-shape. The mounting portion 44 is the part that is attached to the ball screw shaft 20 of the drive assembly 10 and is integrally formed with the bar portion 43. The bar portion 43 extends parallel to the central axis Ax of the ball screw shaft 20, is partially located inside the housing 41, and is supported by the housing 41. As shown in Figure 3, the mounting portion 44 has a wider width than the bar portion 43. The cover 45 closes the upper opening of the housing 41 in the diagram.

[0044] As shown in Figure 3, guide rails 48 are attached to both sides of the bar portion 43 of the slider 42. Each guide rail 48 extends parallel to the longitudinal direction of the bar portion 43 and is fixed to the bar portion 43, for example, by screws 49. Each guide rail 48 is positioned to extend with a high degree of parallelism to the longitudinal direction of the bar portion 43.

[0045] Multiple linear guides 46 are arranged inside the housing 41. The linear guides 46 are fixed to the housing 41, for example, by screws 50. The linear guides 46 support guide rails 48 so that the slider 42 can move linearly along the longitudinal direction of the bar section 43.

[0046] As described above, the mounting portion 44 of the slider 42 is attached to the end portion 20b of the ball screw shaft 20. Therefore, as the hollow rotor 12b, hollow rotating shaft 13, and ball screw nut 18 of the hollow motor 12 rotate, the ball screw shaft 20 moves linearly along the central axis Ax, and the slider 42 also moves linearly together with the ball screw shaft 20. That is, as shown by dashed lines in Figures 1 and 2, as the ball screw shaft 20 moves forward in the forward direction protruding from the housing 11 as indicated by arrow A, the slider 42 also moves forward in the forward direction protruding from the housing 41. As the hollow rotor 12b and ball screw nut 18 of the hollow motor 12 rotate in the opposite direction, the ball screw shaft 20 moves backward in the reverse direction toward the inside of the housing 11, opposite to arrow A, and the slider 42 also moves backward in the reverse direction toward the inside of the housing 41.

[0047] Multiple through holes 43a are formed at the end of the bar portion 43. The through holes 43a are, for example, screw holes, and an end effector (not shown) is attached to the bar portion 43 by the through holes 43a and screws (not shown). Therefore, the end effector, together with the slider 42, is moved in a straight line along the central axis Ax of the ball screw shaft 20.

[0048] It is desirable that the central axis Ax of the ball screw shaft 20 and the direction of movement of the slider 42 are parallel to each other. Since the housing 41, which houses the linear guide 46, is positioned with high precision and fixed to the housing 11 of the drive assembly 10, the longitudinal direction (direction of movement) of the slider 42 is adjusted to be parallel with high precision to the central axis Ax of the ball screw shaft 20.

[0049] As described above, the ball screw nut 18 has a cylindrical portion 18a that is inserted into the front end of the hollow rotating shaft 13, and a flange 18b integrally connected to the front end of the cylindrical portion 18a (see Figure 2). The flange 18b is concentric with the cylindrical portion 18a and extends outward from the cylindrical portion 18a.

[0050] As shown in Figures 4 to 6, the flange 18b of the ball screw nut 18 has a plurality of through grooves 30. Each through groove 30 penetrates the flange 18b in its thickness direction. Each through groove 30 has a U-shape and opens at the outer edge of the flange 18b. These through grooves 30 are arranged at equal angular intervals around the central axis Ax. In the illustrated example, there are 12 through grooves 30 and the angular interval is 30 degrees. Preferably, there are more than 8 through grooves 30 and the angular interval is less than 45 degrees.

[0051] Rotation restricting screws 24 are inserted into the through grooves 30 to restrict the rotation of the ball screw nut 18 around the central axis Ax. In the illustrated example, four rotation restricting screws 24 are used, and the number of through grooves 30 is greater than the number of rotation restricting screws 24. Therefore, the rotation restricting screws 24 are inserted into one of the through grooves 30. The rotation restricting screws 24 are arranged at equal angular intervals (e.g., 90 degrees). Therefore, in the illustrated example, of the twelve through grooves 30 spaced 30 degrees apart, rotation restricting screws 24 are inserted into four through grooves 30 spaced 90 degrees apart.

[0052] As described above, a ring (lock screw) 22 is positioned around the front end of the hollow rotating shaft 13 (see Figure 2). The ring 22 is positioned between the flange 18b of the ball screw nut 18 and the bearing 17, and is screwed onto the male thread 13a at the front end of the hollow rotating shaft 13.

[0053] As shown in Figures 4, 5, and 7, the ring 22 has a large-diameter portion 22a and a small-diameter portion 22b that are concentric with each other. Female threads 22c are formed on the inner circumferential surfaces of the large-diameter portion 22a and the small-diameter portion 22b, which are screwed onto the male thread 13a of the hollow rotating shaft 13.

[0054] As shown in Figure 7, the large-diameter portion 22a has multiple screw holes (insertion holes) 32 and multiple through holes 33. The screw holes 32 and through holes 33 penetrate the large-diameter portion 22a.

[0055] Multiple grooves 34 and 35 are formed in the small-diameter portion 22b. The grooves 34 and 35 open at the outer edge of the small-diameter portion 22b. Each groove 34 communicates with a screw hole 32, and each groove 35 communicates with a through hole 33.

[0056] A rotation restricting screw 24 is screwed into the screw hole 32 to restrict the rotation of the ball screw nut 18 relative to the ring 22. The rotation restricting screw 24 passes through the groove 34. In the illustrated example, four rotation restricting screws 24 are used, and the number of screw holes 32 (4) is the same as the number of rotation restricting screws 24.

[0057] Figure 8 shows the combination of the ball screw nut 18 from Figure 6 and the ring 22 from Figure 7. As shown in Figure 8, each rotation restricting screw 24 passes through one of the through grooves 30 of the flange 18b of the ball screw nut 18 and is screwed into the threaded hole 32 of the ring 22.

[0058] In this way, as shown in Figure 2, the rotation restricting screw 24 fixes the ball screw nut 18 to the ring 22 and also fixes, i.e., connects, the ball screw nut 18 to the hollow rotating shaft 13.

[0059] Figure 9 shows a ring (lock screw) 220 according to another embodiment. In the ring 220, a plurality of screw holes (insertion holes) 32a, 32b and a plurality of through holes 33 are formed in the large diameter portion 22a. The screw holes 32a, 32b and the through holes 33 penetrate the large diameter portion 22a.

[0060] Multiple grooves 34a, 34b, and 35 are formed in the small-diameter portion 22b. The grooves 34a, 34b, and 35 open at the outer edge of the small-diameter portion 22b. Each groove 34a communicates with a screw hole 32a, each groove 34b communicates with a screw hole 32b, and each groove 35 communicates with a through hole 33.

[0061] Rotation restricting screws 24 are screwed into the screw holes 32a and 32b to restrict the rotation of the ball screw nut 18 relative to the ring 220. The rotation restricting screws 24 pass through grooves 34a and 34b. In the illustrated example, four rotation restricting screws 24 are used, and the total number of screw holes 32a and 32b (8) is greater than the number of rotation restricting screws 24.

[0062] In the illustrated example, four screw holes 32a and four screw holes 32b are formed in the large diameter portion 22a. The four screw holes 32a form one set, and the four screw holes 32b form another set. The number of screw holes in each set is the same as the number of rotation-restricting screws 24. The screw holes in each set are arranged at equal angular intervals around the central axis (in the illustrated example, the screw holes 32a are spaced at 90-degree angles, and the screw holes 32b are also spaced at 90-degree angles). Thus, the rotation-restricting screws 24 are screwed into either the screw holes 32a or 32b.

[0063] Figures 10 and 11 show the combination of the ball screw nut 18 from Figure 6 and the ring 220 from Figure 7. As shown in Figures 10 and 11, each rotation restrictor screw 24 passes through one of the through grooves 30 in the flange 18b of the ball screw nut 18 and is screwed into either the screw hole 32a or 32b of the ring 220. In Figure 10, the rotation restrictor screw 24 is screwed into the screw hole 32a, and in Figure 11, the rotation restrictor screw 24 is screwed into the screw hole 32b.

[0064] In this way, as shown in Figure 2, the rotation restricting screw 24 fixes the ball screw nut 18 to the ring 220 and also fixes, i.e., connects, the ball screw nut 18 to the hollow rotating shaft 13.

[0065] The sets of screw holes 32a are arranged at an angular distance of 22.5 degrees around the central axis Ax relative to the sets of screw holes 32b. Preferably, the sets of screw holes 32a are arranged at an angular distance of less than 45 degrees around the central axis Ax relative to the sets of screw holes 32b.

[0066] In the ring 22 in Figure 7 and the ring 220 in Figure 9, the through-hole 33 and groove 35 are used to guide the jig during the assembly of the drive assembly 10. That is, the jig is inserted into the through-hole 33 and engaged with the groove 35. The rotation-restricting screw 24 is not engaged with the through-hole 33 and groove 35.

[0067] In actuator 1, the direction of movement of the slider 42, which is a linear motion member, is adjusted to be parallel to the central axis Ax of the ball screw shaft 20 with high precision. Furthermore, the machining precision of the ball screw shaft 20 and the mounting precision of the ball screw shaft 20 and slider 42 are also high. However, the drive assembly 10 contains factors that reduce the positioning precision of the ball screw shaft 20 and, consequently, the slider 42, as described below.

[0068] First, in each of the bearings 15, 16, and 17 supporting the hollow rotating shaft 13 connected to the ball screw nut 18, the hollow rotating shaft 13 flexes slightly due to errors in the roundness of the inner ring and the outer ring to which the hollow rotating shaft 13 is fixed. As a result, during one rotation of the ball screw nut 18 and the hollow rotating shaft 13, the inner ring moves slightly relative to the outer ring along the direction of the central axis Ax, and the ball screw nut 18 fixed to the hollow rotating shaft 13 to which the inner ring is fixed also moves slightly relative to the outer ring along the direction of the central axis Ax. The amount of movement of the inner ring and the ball screw nut 18 relative to the outer ring can be called the axial error of the ball screw nut 18 due to the bearing. Because the inner ring moves periodically relative to the outer ring, the axial error of the ball screw nut 18 due to the bearing fluctuates periodically (the period corresponds to one rotation of the ball screw nut 18). If there is an axial error in the ball screw nut 18 caused by the bearing, the actual amount of movement of the ball screw shaft 20 inside the ball screw nut 18 will deviate from the ideal amount of movement.

[0069] Figure 12 shows the variation in the axial error of the ball screw nut 18 due to a single bearing with respect to the rotation angle of the ball screw shaft 20. In other words, Figure 12 shows the variation in the amount of movement of the inner ring relative to the outer ring of a single bearing with respect to the rotation angle of the ball screw shaft 20. Over the period of one rotation of the ball screw nut 18, the axial error of the ball screw nut 18 due to the bearing varies in a nearly sinusoidal manner. Since the axial error due to the bearing is caused by errors in the roundness of the inner ring and the roundness of the outer ring, it is extremely difficult to eliminate it completely.

[0070] Furthermore, in a ball screw mechanism having a ball screw shaft 20 that is moved linearly by the rotation of a ball screw nut 18, the ball screw shaft 20 deflects slightly due to misalignment and machining errors between the ball screw nut 18 and the ball screw shaft 20. Due to this deflection, the ball screw shaft 20 moves a small amount along the direction of the central axis Ax relative to the ball screw nut 18. Therefore, the actual amount of movement of the ball screw shaft 20 during one rotation of the ball screw nut 18 deviates from the ideal amount of movement of the ball screw shaft 20. This deviation can be called the axial error of the ball screw shaft 20 caused by the ball screw mechanism. While the ball screw shaft 20 is continuously moved, the axial error of the ball screw shaft 20 caused by the ball screw mechanism fluctuates periodically (the period corresponds to one rotation of the ball screw nut 18).

[0071] Figure 13 shows the variation in the axial error of the ball screw shaft 20 due to the ball screw mechanism with respect to the rotation angle of the ball screw shaft 20. The axial error of the ball screw shaft 20 fluctuates in a nearly sinusoidal manner for the period of one rotation of the ball screw nut 18. Since the axial error due to the ball screw mechanism is caused by misalignment and machining errors between the ball screw nut 18 and the ball screw shaft 20, it is extremely difficult to completely eliminate it.

[0072] The displacement of the ball screw shaft 20 is affected by the axial error caused by the bearing and the axial error caused by the ball screw mechanism. In other words, the displacement of the ball screw shaft 20 is the sum of the axial error caused by the bearing and the axial error caused by the ball screw mechanism.

[0073] If the periodic axial error caused by the ball screw mechanism is amplified by the periodic axial errors caused by bearings 15, 16, and 17, the actual amount of movement of the ball screw shaft 20 will deviate significantly from the ideal amount of movement, reducing the positioning accuracy of the ball screw shaft 20 and, consequently, the slider 42. Figure 14 shows the fluctuation of the total axial error, where the phase of the axial error caused by the ball screw mechanism in Figure 13 coincides with the phase of the axial error caused by the bearings in Figure 12, resulting in the worst possible amplification of the axial errors in Figure 12 and Figure 13.

[0074] However, when the periodic axial error caused by the ball screw mechanism cancels out the periodic axial error caused by the bearings 15, 16, and 17, the actual amount of movement of the ball screw shaft 20 approaches the ideal amount of movement, improving the positioning accuracy of the ball screw shaft 20 and, consequently, the slider 42. Figure 15 shows the fluctuation of the total axial error, where the phase of the axial error caused by the ball screw mechanism in Figure 13 is shifted by 180 degrees from the phase of the axial error caused by the bearings in Figure 12, and the axial errors in Figure 12 and Figure 13 are ideally canceled out.

[0075] Therefore, if the axial errors of the ball screw nut 18 caused by the bearings 15, 16, and 17 and the axial errors of the ball screw shaft 20 caused by the ball screw mechanism can be offset as much as possible, the total axial error of the ball screw shaft 20 should be reduced as a result.

[0076] In reality, the drive assembly 10 has three bearings 15, 16, and 17, in which the inner ring moves relative to the outer ring. The axial error of the ball screw nut 18 due to the three bearings 15, 16, and 17 is the sum of the axial error components of the ball screw nut 18 due to each bearing. However, if the axial error component with the largest amplitude among these can be offset against the axial error of the ball screw shaft 20 due to the ball screw mechanism, the total axial error of the ball screw shaft 20 should be able to be reduced. It is most preferable that the phase of the axial error due to the ball screw mechanism is shifted by 180 degrees from the phase of the axial error component with the largest amplitude among the axial errors due to the bearings 15, 16, and 17, but if that is difficult, it is preferable that the shift is close to 180 degrees.

[0077] In the drive assembly 10, the ring 22 or 220 is restricted from relative rotation to the inner ring of the bearing 17. Therefore, influenced by the periodic movement of the inner ring relative to the outer ring of the bearings 15, 16, and 17, the ring 22 or 220 moves periodically along the central axis Ax relative to the housing 11. The axial movement of the ring 22 or 220 is synchronized with the movement of the ball screw nut 18 fixed to the hollow rotating shaft 13. The same axial error as that of the ball screw nut 18 caused by the bearings 15, 16, and 17 is imparted to the ring 22 or 220.

[0078] On the other hand, the ball screw nut 18 is the reference position for the axial error of the ball screw shaft 20 caused by the ball screw mechanism.

[0079] Therefore, by adjusting the relative angular position of the ball screw nut 18 with respect to the ring 22 or 220 around the central axis Ax, the axial error of the ball screw shaft 20 caused by the bearings 15, 16, and 17 and the axial error of the ball screw shaft 20 caused by the ball screw mechanism can be offset as much as possible.

[0080] However, as shown in Figure 16, if there are four through grooves 30 formed in the flange 18b of the ball screw nut 18 (the angular spacing of the through grooves 30 is 90 degrees) and four screw holes 32 formed in the ring 22 (the angular spacing of the screw holes 32 is 90 degrees), then the ball screw nut 18 can only be adjusted in 90-degree increments relative to the ring 22. Therefore, it is difficult to reduce the total axial error because, in addition to adjusting the phase of the axial error caused by the ball screw mechanism in 90-degree increments relative to the phase of the axial error caused by the bearings 15, 16, and 17, it is only possible to adjust the phase of the axial error caused by the ball screw mechanism in 90-degree increments.

[0081] For example, in Figure 16, assume that the axial error of the ball screw nut 18 caused by bearings 15, 16, and 17 is maximum at angular position α. ​​Also assume that the axial error of the ball screw shaft 20 caused by the ball screw mechanism is maximum at angular position β1. By adjusting the angular position of the ball screw nut 18 relative to the ring 22 by 90 degrees, 180 degrees, and 270 degrees, the angular position where the axial error of the ball screw shaft 20 caused by the ball screw mechanism is maximum can be changed to β2, β3, and β4. Of the angular positions β1 to β4, angular position β3 is the furthest from angular position α, so it may have a high effect in canceling out the axial error. However, in the example in Figure 16, the phase of the axial error caused by the ball screw mechanism can only be adjusted in 90-degree increments, i.e., in four steps, relative to the phase of the axial error caused by bearings 15, 16, and 17.

[0082] In contrast, in the flange 18b of the ball screw nut 18 shown in Figure 6, the angular spacing of the through groove 30 is less than 45 degrees. Therefore, as is clear from Figure 8, the ball screw nut 18 can be adjusted relative to the ring 22 at angular intervals smaller than 45 degrees (30 degrees in the illustrated example). That is, the phase of the axial error caused by the ball screw mechanism can be adjusted at angular intervals smaller than 45 degrees relative to the phase of the axial error caused by the bearings 15, 16, and 17, so that the total axial error can be further reduced. In the illustrated example, the phase of the axial error caused by the ball screw mechanism can be adjusted at 30-degree intervals (i.e., 12 steps) relative to the phase of the axial error caused by the bearings 15, 16, and 17.

[0083] The worker manufacturing actuator 1 fixes the ball screw nut 18 to the ring 22 using the rotation restricting screw 24 at the angular position where the periodic fluctuation of the displacement of the ball screw shaft 20 relative to the ball screw nut 18 (axial error caused by the ball screw mechanism) most effectively cancels out the periodic movement of the inner ring relative to the outer ring of the bearings 15, 16, 17 (axial error caused by the bearings 15, 16, 17). As a result, the total axial error applied to the ball screw shaft 20 can be minimized.

[0084] For example, first, the axial error Meas1(i) caused by the ball screw mechanism is measured for multiple angular positions (i). Next, we determine the estimation formula Est1(i) for the axial error caused by the ball screw mechanism for multiple angular positions (i). The estimation formula Est1(i) is determined by determining the variables of the theoretical formula Est(i), which has multiple variables. From the measured axial error Meas1(i), we use the least squares method to calculate Σ(Meas1(i)-Est(i)). 2 By determining these variables in such a way that the given value is minimized, the estimation formula Est1(i) for the axial error caused by the ball screw mechanism is determined.

[0085] Similarly, the axial error Meas2(i) caused by bearings 15, 16, and 17 is measured for multiple angular positions (i). Next, for multiple angular positions (i), we determine the estimation formula Est2(i) for the axial error caused by bearings 15, 16, and 17. The estimation formula Est2(i) is also determined by determining the variables of the theoretical formula Est(i). From the measured axial error Meas2(i), we use the least squares method to calculate Σ(Meas2(i)-Est(i)). 2 By determining these variables in such a way that the given value is minimized, the estimation formula Est2(i) for the axial error caused by bearings 15, 16, and 17 is determined.

[0086] Furthermore, within the limitations of the adjustable angle range, the relative angle of the ball screw nut 18 with respect to the ring 22 is determined such that the axial error on estimation formula Est1(i) most effectively cancels out the axial error on estimation formula Est2(i). The ball screw nut 18 is then fixed to the ring 22 according to this relative angle.

[0087] Furthermore, if the ring 220 shown in Figure 9 is used instead of the ring 22 shown in Figure 7, and the ring 220 is combined with the flange 18b of the ball screw nut 18 shown in Figure 6, then, as shown in Figures 10 and 11, there is an option to use either the set of screw holes 32a or the set of screw holes 32b to fasten the rotation-restricting screw 24. Since the angular spacing between the set of screw holes 32a and the set of screw holes 32b is different from the angular spacing of the through groove 30, the ball screw nut 18 can be adjusted to smaller angular intervals relative to the ring 220. Therefore, the total axial error can be further reduced. In the illustrated example, the set of screw holes 32a and the set of screw holes 32b are separated by 22.5 degrees. Therefore, the adjustable angular pitch is 7.5 degrees, which is the difference between the angular spacing of the through groove 30 and 22.5 degrees. In this way, the ball screw nut 18 can be adjusted to 7.5-degree intervals relative to the ring 220. In other words, the phase of the axial error caused by the ball screw mechanism can be adjusted in 7.5-degree increments relative to the phase of the axial error caused by bearings 15, 16, and 17.

[0088] Figure 17 is a graph showing the variation in the amplified total axial error and the offset total axial error with respect to the displacement of the ball screw shaft 20. In Figure 17, curve C2 shows an example of the variation in the amplified total axial error. This is the result of assembling the drive assembly 10 without considering the angular position of the ball screw nut 18 relative to the ring 22 or 220.

[0089] In Figure 17, curve C1 shows an example of the variation in the reduced total axial error. This is the result of adjusting the angular position of the ball screw nut 18 relative to the ring 22 or 220 to assemble the drive assembly 10 in a way that minimizes the total axial error.

[0090] As is evident from the comparison of curves C1 and C2, the total axial error can be significantly reduced by adjusting the angular position of the ball screw nut 18 relative to the ring 22 or 220.

[0091] Let's explain a specific example of reducing the total axial error. In the following explanation, we assume a ball screw designed so that the ball screw shaft 20 moves 1 mm along the central axis Ax for one rotation of the ball screw nut 18. We also assume that the axial error due to bearings 15, 16, and 17 is a maximum of 1 μm during one rotation of the ball screw nut 18, and the axial error due to the ball screw mechanism is also a maximum of 1 μm. In this case, the worst-case amplified total axial error is 2 μm, and the best-case reduced total axial error is 0 μm.

[0092] Let's explain using the combination in Figure 16 as an example. In the example in Figure 16, the angular spacing of the four through grooves 30 in the flange 18b of the ball screw nut 18 is 90 degrees. Assume that the axial error of the ball screw nut 18 caused by bearings 15, 16, and 17 is at its maximum at angular position α. ​​Assume that the axial error of the ball screw shaft 20 caused by the ball screw mechanism is at its maximum at angular position β1. If angular position α and angular position β1 coincide, the total axial error is amplified to the worst extent, and if angular position α and angular position β1 are separated by 180 degrees, the total axial error is reduced to the best extent.

[0093] However, in the example in Figure 16, the phase of the axial error caused by the ball screw mechanism is set at a 90-degree interval relative to the phase of the axial error caused by bearings 15, 16, and 17, meaning only four adjustments are possible. In other words, unless the difference θ between angular position α and angular position β1 is 0 degrees, 90 degrees, 180 degrees, or 270 degrees, adjusting the angular position of the ball screw nut 18 will not eliminate the phase difference of θ degrees. In particular, when the difference θ is half the angular spacing of the through groove 30, the total axial error will be large even if the angular position of the ball screw nut 18 is adjusted. In the example in Figure 16, the angular spacing of the through groove 30 is 90 degrees, so the difference θ that results in a large total axial error is 45 degrees. In this case, the maximum total axial error E that can occur is calculated by the following formula. E = (2 × (1 - cosθ)) 1 / 2 =(2 × (1 - cos45°)) 1 / 2 =0.77

[0094] Although not shown in the diagram, assume that eight through grooves 30 are formed in the flange 18b of the ball screw nut 18. In this case, the angular spacing of the through grooves 30 is 45 degrees, so the difference θ that results in a large total axial error is 22.5 degrees. In this case, the maximum total axial error E that can occur is calculated using the following formula. E = (2 × (1 - cosθ)) 1 / 2 =(2 × (1 - cos 22.5°)) 1 / 2 =0.39

[0095] In the case of the combination shown in FIG. 8, twelve through grooves 30 are formed in the flange 18b of the ball screw nut 18. In this case, since the angular pitch between the through grooves 30 is 30 degrees, the difference θ in the total axial error, which is large, is 15 degrees. In this case, the maximum total axial error E that can occur is calculated by the following formula. E = (2 × (1 - cos θ)) 1 / 2 = (2 × (1 - cos 15°)) 1 / 2 = 0.26 In this case, compared with the example of FIG. 16, the maximum total axial error E that can occur can be reduced to about 1 / 3.

[0096] In the case of the combination shown in FIGS. 10 and 11, the ring 220 further has sets of screw holes 32a and sets of screw holes 32b. In this case, since the adjustable angular pitch is 7.5 degrees, the difference θ in the total axial error, which is large, is 3.75 degrees, which is half of the angular pitch. In this case, the maximum total axial error E that can occur is calculated by the following formula. E = (2 × (1 - cos θ)) 1 / 2 = (2 × (1 - cos 3.75°)) 1 / 2 = 0.07 In this case, compared with the example of FIG. 16, the maximum total axial error E that can occur can be reduced to less than ¼.

[0097] In the above embodiment, a male screw 13a centered on the central axis Ax is formed at the front end of the hollow rotary shaft 13, and a female screw 22c screwed onto the male screw 13a is formed on the ring 22 or 220. When the male screw 13a and the female screw 22c are screwed together and the ring 22 or 220 is pressed against the inner ring of the bearing 17, relative rotation of the ring 22 with respect to the hollow rotary shaft 13 and the inner ring of the bearing 17 is restricted. Therefore, the ring 22 or 220 can be made non-rotatable without using other fixtures.

[0098] Although the present invention has been illustrated and described above with reference to preferred embodiments, those skilled in the art will understand that modifications to form and detail are possible without departing from the scope of the invention as described in the claims. Such modifications, alterations, and changes should be included within the scope of the present invention.

[0099] For example, in the above embodiment, the hollow rotating shaft 13 is supported by three bearings 15, 16, and 17, but the number of bearings supporting the hollow rotating shaft 13 is arbitrary.

[0100] In the above embodiment, the flange 18b of the ball screw nut 18 has a plurality of through grooves 30 formed at angular intervals around the central axis Ax. However, as shown in Figure 18, through holes 36 may be formed instead of through grooves 30. That is, the flange 18b of the ball screw nut 18 may have a plurality of through holes 36 formed at angular intervals around the central axis Ax.

[0101] In the above embodiment, a rotation restricting screw 24 is screwed into the threaded hole 32 of the large-diameter portion 22a of the ring 22, or into 32a or 32b of the large-diameter portion 22a of the ring 220, to restrict the rotation of the ball screw nut 18 relative to the ring 22 or ring 220. However, as shown in Figure 19, a flat ring (lock screw) 230 may be provided instead of the ring 22 or ring 220. The inner circumferential surface of the ring 220 has a female thread 230c that is screwed into the male thread 13a formed at the front end of the hollow rotating shaft 13. The ring 230 has multiple through holes (insertion holes) 230a formed at angular intervals around the central axis Ax, instead of screw holes 32, 32a, 32b. Each of the rotation restricting screws 24 passes through one of the multiple through grooves 30 (or through holes 36) of the flange 18b of the ball screw nut 18 and is inserted into one of the through holes 230a of the ring 230. Nuts 231 are fastened to the ends of each rotation restricting screw 24. In this way, the rotation restricting screws 24 fix the flange 18b of the ball screw nut 18 to the ring 230 and restrict the rotation of the ball screw nut 18 relative to the ring 230.

[0102] As shown in Figure 20, instead of the rotation restricting screws 24, a retaining pin (rotation restricting member) 232 may be provided. Each retaining pin 232 is inserted into one of the multiple through holes 230a of the ring 230, from the bearing 17 side toward the cover 11c side, and passes through one of the multiple through holes 36 (or through grooves 30) of the flange 18b of the ball screw nut 18. A retaining ring 233 is fixed to the end of each retaining pin 232. In this way, the retaining pins 232 fix the flange 18b of the ball screw nut 18 to the ring 230, thereby restricting the rotation of the ball screw nut 18 relative to the ring 230. [Explanation of symbols]

[0103] Ax...Central axis, 1...Actuator, 10...Drive assembly, 11...Housing, 12...Hollow motor, 12a...Stator, 12b...Hollow rotor, 13...Hollow rotating shaft, 13a...Male thread, 15,16,17...Bearing, 18...Ball screw nut, 18a...Cylindrical section, 18b...Flange, 20...Ball screw shaft, 22,220,230...Ring (lock screw), 22c,230c...Female thread, 24...Rotation restricting screw (rotation restricting member), 30...Through groove, 32,32a,32b...Threaded hole (insertion hole), 36...Through hole, 42...Slider (linear motion member), 230a...Through hole (insertion hole), 232...Retaining pin (rotation restricting member)

Claims

1. A hollow motor having a hollow rotating shaft, a hollow rotor fixed to the hollow rotating shaft, and a stator arranged around the hollow rotor to rotate the hollow rotor and the hollow rotating shaft around a central axis, A bearing comprising an outer ring fixed to a housing and an inner ring fixed to the hollow rotating shaft, and having at least one bearing that rotatably supports the hollow rotating shaft, A ball screw nut is provided, which is concentrically positioned with the hollow rotating shaft and has a cylindrical portion inserted into the hollow rotating shaft, and a flange connected to the cylindrical portion, and is connected to the hollow rotating shaft and rotates together with the hollow rotating shaft around the central axis. A ball screw shaft that is engaged with the ball screw nut and is moved in a straight line by the rotation of the ball screw nut, The hollow rotating shaft and the ring that restricts relative rotation with respect to the inner ring of the bearing, The flange of the ball screw nut is fixed to the ring, and a plurality of rotation restricting members restrict the rotation of the ball screw nut relative to the ring. Equipped with, The relative angular position of the ball screw nut with respect to the ring is adjustable around the central axis. The flange of the ball screw nut has a plurality of through holes or through grooves formed at angular intervals around the central axis, the angular interval being less than 45 degrees, and the number of through holes or through grooves is greater than the number of rotation restricting members. The ring has a plurality of insertion holes formed at angular intervals around the central axis, Each of the rotation restricting members passes through either the through hole or through groove of the flange and is inserted into either of the insertion holes of the ring, thereby fixing the flange of the ball screw nut to the ring. An actuator characterized by the following features.

2. The number of insertion holes in the ring is greater than the number of rotation restricting members, the insertion holes form multiple sets, the insertion holes in each set are arranged at equal angular intervals around the central axis, and the multiple sets are arranged around the central axis at angular intervals different from the angular intervals of the through holes or through grooves. The actuator according to claim 1, characterized by the features described above.

3. A male thread is formed at the end of the hollow rotating shaft, with the central axis as the center. The ring has an internal thread that is screwed onto the external thread, As the male thread and female thread are screwed together and pressed against the inner ring, the relative rotation of the ring is restricted with respect to the hollow rotating shaft and the inner ring. The actuator according to claim 1 or 2, characterized in that it is as described above.

4. The actual amount of movement of the ball screw shaft along the central axis relative to the ball screw nut during rotation differs from the ideal amount of movement of the ball screw shaft relative to the ball screw nut, and this difference between the ideal amount of movement and the actual amount of movement fluctuates periodically with the rotation of the ball screw nut. As the hollow rotating shaft fixed to the inner ring of the bearing rotates, the inner ring of the bearing periodically moves along the central axis relative to the outer ring of the bearing. The ball screw nut is fixed to the ring at an angular position where the periodic fluctuations in the displacement of the ball screw shaft most effectively cancel out the periodic movement of the inner ring relative to the outer ring of the bearing. The actuator according to claim 1 or 2, characterized in that it is as described above.

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